In the first two years of this project we performed exhaustive studies that ultimately led us to conclude that only a small subset (2-3%) of familial Alzheimer disease (FAD) kindreds possess mutations in the gene encoding the Abeta peptide precursor (APP). This has been confirmed by other large-scale studies performed around the world. These data along with other family studies have shown that FAD is clearly heterogeneous. For example, linkage studies using markers from other chromosomes have led to the localization of a late-onset (> 65 years) FAD locus mapping within or close to the APOE gene on chromosome 19. More recently, our extensive FAD pedigree collection that we have amassed over the past ten years has allowed our laboratory in collaboration with Dr. Peter Hyslop in Toronto, to localize a major early-onset FAD gene on chromosome l4. Similar findings were initially reported by Schellenberg and colleagues and later confirmed by our group as well as those of Mullan and Van Broeckhoven. Among the pedigrees screened in our Boston-Toronto study, six independent pedigrees individually yielded lod scores exceeding the conventional significance value of 3. The magnitude and robustness of these scores far exceeds any lod scores achieved on chromosome 2l or l9, and indicates definitively that there is a chromosome l4 locus that most likely accounts for a major proportion (70-90%) of early-onset FAD (<65 years). In view of these combined findings, we have chosen to spend a major portion of year 03 of this project in attempting to isolate the chromosome 14 FAD gene, and have already made substantial progress toward this goal. Having accomplished the primary goal of the originally proposed studies on the APP gene, we propose to re-channel the funding for this project toward the exceedingly important task of isolating and preliminarily characterizing the chromosome 14 FAD gene defect. Therefore, the goal of the current proposal is to employ a variety of strategies including positional cloning, physical mapping, and testing of candidate genes to ultimately isolate, identify, and provide a preliminary characterization of the FAD gene defect on chromosome l4.